This is a revised application to renew our program of research on neurofilaments, which are the intermediate filaments of neurons. Neurofilaments are the most abundant structure in large axons and their principal function is to increase axonal caliber, which is a critical determinant of axonal conduction velocity. Neurofilaments are also of clinical interest because neurofilament protein mutations can cause peripheral neuropathy and because neurofilaments accumulate abnormally in many neurodegenerative diseases. Though neurofilaments are structural components of axons, we have demonstrated that they are also cargoes of axonal transport. The filaments move rapidly but the overall rate is slow because they spend most of their time pausing. Studies on laboratory animals have shown that neurofilament accumulations can be caused by an impairment of neurofilament transport. Thus our long-term goal is to understand the mechanism of neurofilament transport and how it is regulated in health and disease. In the past few years we have discovered two remarkable behaviors for neurofilaments, which is that they can lengthen by fusing end-to-end and that they can also be shortened by a severing mechanism. We are excited about these findings because they suggest a novel mechanism for the regulation of neurofilament length that has implications for the regulation of neurofilament transport as well as intermediate filament dynamics in other cell types. We have developed tools and strategies to analyze the mechanism of neurofilament severing and the influence of neurofilament length on neurofilament transport in axons, and we have also obtained evidence that neurofilament motors may form special associations with the leading ends of these polymers. The current proposal builds on this progress to address three aims: In Aim 1 we will test the hypothesis that neurofilament severing is a robust and efficient phenomenon in neurons and we will test a specific hypothesis for the severing mechanism. These experiments will establish the prevalence of neurofilament severing, which is a novel phenomenon not previously described for intermediate filaments, and they will elucidate a novel mechanism for the regulation of neurofilament length. In Aim 2 we will test the hypothesis that motor proteins form special attachments with the leading ends of neurofilaments. These experiments will identify a novel mechanism for the interaction of motors with intermediate filaments that has intriguing implications for the mechanism and regulation of the movement of these unique intracellular cargoes. In Aim 3 we will test the hypothesis that neurofilament transport is regulated by neurofilament length. These experiments will define a novel mechanism for the regulation of neurofilament transport and they will establish the functional significance of neurofilament severing and annealing in axons. Overall this project will elucidate novel aspects of neurofilament dynamics in axons that may represent targets for future therapeutic intervention to alleviate neurofilament accumulations or alterations in disease.